1. Field of the Invention
The present invention relates to a torque detecting apparatus for monitoring the amount of torque imposed on a torque-transmitting shaft.
2. Description of the Prior Art
In the fields of motor vehicles, ships, airplanes, and various machine tools, deformation of components in the course of operation must be nondestructively measured with high precision. In order to satisfy such needs, various types of means for detecting the amount of torque imposed on the torque-transmitting shafts or the like have been proposed.
A typical basic conventional device used in torque measurement is a strain gauge, exemplified by a metal-film strain gauge or a semiconductor strain gauge. Either gauge utilizes a change in the electrical resistance caused by deformation of a component when a stress load is imposed thereon. The change in electrical resistance is detected, and hence the imposed stress is detected. However, since a change in the electrical resistance of the metal-film strain gauge is very small, a detector having a high gain is required so as to obtain practical detecting sensitivity. In the case of the semiconductor strain gauge, the change in electrical resistance tends to fluctuate, and thus, the detecting accuracy and detecting stability are undesirably degraded.
In order to solve the above problems associated with strain gauges, a torque detecting apparatus utilizing the magnetoelastic effect of a magnetic metal material having a large magnetostrictive effect is proposed (Papers Tec. Meet. Magnetics, IEEJ, MAG-81-72). The principle of the above torque detecting apparatus will be briefly described.
Referring to FIG. 1, reference numeral 1 denotes a torque-transmitting shaft subjected to torque detection. Thin annular strip 2 of an amorphous magnetic alloy is wound around the torque-transmitting shaft, and is fixed thereon. Strip 2 has induced magnetic anisotropy Ku' 4 in a direction at an inclined angle .theta. with respect to circumferential direction 3. For illustrative convenience, conditions .theta.&gt;45.degree., and saturated magnetostriction constant (.lambda.s)&gt;0 are established. The magnetic material constituting strip 2 is selected from a material exhibiting soft magnetism, such as an amorphous magnetic alloy, Permalloy (an Fe--Ni alloy), or Sendust (an Fe--Al--Si alloy).
When torque 5 acts on torque-transmitting shaft 1, the stress generated by shaft 1 is transmitted to thin annular strip 2. Tension .sigma. is generated in strip 2, in a +45.degree. direction. At the same time, compressive stress -.sigma. is generated in the -45.degree. direction. The magnetostrictive effect of the stress generates induced magnetic anisotropy Ku" 6 directed toward the +45.degree. direction with respect to the circumferential direction of strip 2. The magnitude of Ku" 6 is represented by Ku" 6=3.lambda.s.sigma..
The total magnetic anisotropy exhibited by thin annular strip 2 is changed to the total force of the preacted magnetic anisotropy Ku' 4 and stress-induced magnetic anisotropy Ku" 6 generated by the magnetostrictive effect, that is, Ku 7 in FIG. 1. By detecting the change in magnetic anisotropy, the stress generated in the thin annular strip, i.e., the torque imposed on the torque-transmitting shaft, can be detected.
In a conventional apparatus, a means for detecting a change in the magnetic anisotropy of thin annular strip 2 usually comprises a detection coil. The functioning of the detection coil will now be explained.
In general, magnetic permeability .mu. is changed in accordance with the direction of induced magnetic anisotropy, with respect to the direction of magnetic excitation. If the magnetic anisotropy of the thin magnetic strip is changed, magnetic flux density B of the strip is changed in accordance with B=.mu.H. When a detection coil (not shown) is arranged near strip 2, the change in magnetic flux density B of strip 2 causes the detection coil to generate an e.m.f. (electromotive force). When the detection coil is connected to a detecting circuit and a change in voltage across the coil terminals is detected, the change in magnetic anisotropy of the thin annular strip, i.e., the magnitude of the torque imposed on the torque-transmitting shaft, can be detected. In the overall torque detecting apparatus, strip 2 and the detection coil serve as a sensor.
In the conventional torque detecting apparatus utilizing the magnetoelastic effect, the secondary sensor, such as the detection coil, is generally arranged so as to be separated from the thin annular strip serving as the primary sensor. Such a noncontact structure can be more easily mounted on the shaft, and does not require the use of any sliding parts which might generate friction. Therefore, the reliability of the torque detecting apparatus is improved.
However, since the secondary sensor is separated from the primary sensor and an air gap is formed therebetween, the intensity of the torque detection signal is lowered, and detecting sensibility is therefore degraded.
Another problem arises when the conventional torque detecting apparatus is applied to a torque transmitting shaft having a large diameter and a high output, such as a rolling mill or a cutter. More specifically, in the torque-transmitting shaft of a large DC motor for driving a rolling mill for heavy industrial use, the rated torque is about 0.8 kg/mm.sup.2. However, in a specific application, such as in the rolling of a steel ingot, the torque is increased to 300 to 600%, i.e., about 5 kg/mm.sup.2. In a conventional noncontact type torque detecting apparatus, the torque in the above specific application cannot be detected with high responsibility. In particular, the very large torque described above appears as torsional oscillation of about 200 Hz, and the conventional torque detecting apparatus cannot respond to such a frequency. Further, the linearity between the loaded torque and the magnetic anisotropy generated by thin annular strip 2 is insufficient. Satisfactory linearity is established only within the range encompassing the lowest torque values. The very large torque as described above falls outside the possible detection range.
Problems presented by the magnetoelastic characteristics of thin annular strip 2 will now be described below.
In the conventional torque detecting apparatus, the magnetic material for strip 2, serving as the primary sensor, comprises one having as large a saturated magnetostriction constant (.lambda.s) as possible (e.g., 30.times.10.sup.-6 or more) because an output produced by magnetoelastic effect is increased to thereby improve the detecting sensitivity. When constant .lambda.s is increased, the detection sensitivity can be improved. However, the linearity range of the output, with respect to the torque, becomes undesirably narrower. For this reason, the conventional torque detecting apparatus can be applied to only a limited, narrow torque range, and cannot be used in the case of a special application, such as a rolling mill or the like.
Thin annular strip 2 is prepared such that a thin ribbon having a predetermined magnetic anisotropy is bent according to the radius of curvature of torque-transmitting shaft 1. Therefore, anisotropy is induced in strip 2 by stress, upon its deformation. The magnetic anisotropy applied to strip 2 in advance, is degraded by the influence of magnetostriction. In fact, the anisotropy, upon deformation of strip 2, increases when saturated magnetostriction constant .lambda.s of the magnetic material constituting strip 2 is large, and torque detecting characteristics are adversely affected.